1. Field of the Invention
The present invention relates to a method for producing high-purity silicon. The high-purity silicon is used for a solar battery.
2. Description of the Related Art
As for silicon to be used for a solar battery, the purity has to be 99.9999 mass % or more, each of the metallic impurities in the silicon is required to be not more than 0.1 mass ppm. Especially, the impurity of boron (B) is required to be not more than 0.3 mass ppm. Although silicon made by the Siemens Process, which is used for a semiconductor, can meet the above requirements, the silicon is not suitable for a solar battery. This is due to the fact that the manufacturing cost of silicon by the Siemens Process is high while a solar battery is required to be inexpensive.
Several methods have been presented in order to produce high-purity silicon at a low cost.
The process of unidirectional solidification of silicon metal has been well known for a long time. In such a process, molten silicon metal is unidirectionally solidified to form a more purified solid phase silicon utilizing the difference in solubility of impurities between solid phase and liquid phase. Such a process can be effectively used for purifying silicon from a variety of metallic impurities. However, this method cannot be used for purifying silicon from boron because the difference in solubility of boron between solid phase and liquid phase is too small to purify silicon from boron.
The process of vacuum melting silicon is also well known. This process removes low boiling point impurities from silicon by holding molten silicon in a vacuum state and is effective to remove carbon impurities from silicon. However, this method cannot be applied to purifying silicon from boron because boron in molten silicon does not normally form a low boiling point substance.
As mentioned above, boron has been thought to be a problematic component because boron in silicon is the most difficult impurity to removed from and yet greatly affects the electrical property of silicon. Methods for which the main purpose is to remove boron from silicon are disclosed as follows.
JP56-32319A discloses a method for cleaning silicon by acid, a vacuum melting process for silicon and a unidirectional solidification process for silicon. Additionally, this reference discloses a purification method using slag for removing boron, where the impurities migrate from the silicon to the slag, which is placed on the molten silicon. In the patent reference JP56-32319A, the partition ratio of boron (concentration of boron in slag/concentration of boron in silicon) is 1.357 and the obtained concentration of boron in the purified silicon is 8 mass ppm by using slag including (CaF2+CaO+SiO2). However, the concentration of boron in the purified silicon does not satisfy the requirement of silicon used for solar batteries.
JP58-130114A discloses a slag purification method, where a mixture of ground crude silicon and slag containing alkaline-earth metal oxides and/or alkali metal oxides are melted together. However, the minimum boron concentration of the obtained silicon is 1 mass ppm, which is not suitable for a solar battery. In addition, it is inevitable that new impurities are added when the silicon is ground, which also makes this method inapplicable to solar batteries.
Non-patent reference, “Shigen to Sozai” (Resource and Material) 2002, vol. 118, p. 497-505, discloses another example of slag purification where the slag includes (Na2O+CaO+SiO2) and the maximum partition ratio of boron is 3.5. The partition ratio 3.5 is the highest value disclosed in the past, however, this slag purification is still inapplicable to solar batteries considering the fact that the boron concentration in the practically available raw material of slag.
As mentioned above, conventional slag purification methods, which fail to obtain a practically available high partition ratio of boron, are not suitable for obtaining silicon useful in a solar battery. The reason why the partition ratio of boron, when purifying silicon from boron, tends to be low is that silicon is oxidized as easily as boron. In slag purification methods, boron in silicon tends to be non-oxidized and the non-oxidized boron is hardly absorbed in the slag. The slag purification method is widely used for removing boron from steel because boron is far more easily oxidized than steel. Because of the essential difference in properties between steel and silicon, the slag purification technique in steel industry cannot simply be applied to removing boron from silicon.
Methods combining conventional slag purification and other methods are presented.
JP2003-12317A discloses another purification method. In this method, fluxes such as CaO, CaO3 and Na2O are added to silicon and they are mixed and melted. Then, blowing oxidizing gas into the molten silicon results in purification. However, silicon purified by this method has a boron concentration of about 7.6 mass ppm, which is not suitable for use in a solar battery. Furthermore, it is difficult, from an engineering point of view, to blow stably oxidizing gas into molten silicon at low cost. Therefore, the method disclosed in JP2003-12317A is not suitable for the purification of silicon.
U.S. Pat. No. 5,972,107 and U.S. Pat. No. 6,368,403 disclose methods for purifying silicon from boron where a special torch is used and water vapor and SiO2 are supplied in addition to oxygen and hydrogen and CaO, BaO and/or CaF2 to molten silicon.
The technologies in U.S. Pat. No. 5,972,107 and U.S. Pat. No. 6,368,403, requiring not only expensive equipments such as a special torch but also a complicated operation, are difficult to implement from an industrial point of view.
The conventional technologies mentioned above can be classified into two categories. The first category includes methods where slag only is supplied onto molten silicon (disclosed in JP56-32319A and JP58-130114A, hereinafter referred to as “simple slag purification method”). The second category includes methods where oxidizing gas is contacted with the molten silicon and slag and/or raw materials of slag such as SiO2 are supplied onto molten silicon (disclosed in JP2003-12317A, U.S. Pat. No. 5,972,107 and U.S. Pat. No. 6,368,403, hereinafter referred to as “complex slag purification method”).
A feature of the “simple slag purification method” is that the partition rate of boron is about 3 or less (normally about 1). Therefore, firstly, in conventional use, the target boron concentration is normally set at 1 mass ppm or more. This is due to the fact that it is inevitable for the slag to contain boron in an amount of 1 mass ppm or more due to the raw materials used for slag. As long as the partition rate of boron is around 1, it is theoretically difficult to obtain silicon having a boron concentration that is much lower than the boron concentration of the slag no matter how much the slag is used. Although it is theoretically possible to reduce the boron concentration by purifying the raw slag materials, it is not industrially feasible due to being economically unreasonable. Secondly, the purification is attempted at one time using a great amount of slag. The amount of slag may be one to several times the amount of molten silicon. This is because the boron concentration of silicon metal is at best 10 mass ppm and the targeted concentration of boron is about 1 mass ppm. Therefore, if the partition rate is around 2, and five times the amount of slag compared to molten silicon is used, silicon with the targeted boron concentration can be obtained in a single purification.
In a “complex slag purification method”, the slag used is granular and the amount of slag added is relatively very small compared to the amount of molten silicon. The removal of boron is performed mainly through vaporization by oxidizing gas. Supplementary used slag does not make so much of an impression that no report of partition rate data of boron remains. Also, the raw slag materials are fed in the form of powder. Therefore, once the materials are turned to slag, they must be discarded after completing the purification process.